I used to use it in a percussion rifle and it was fine.
It didn't seem to produce quite the accuracy as Pyrodex, for some reason, though.
But it still needs to be cleaned out as it causes rust, too.
I inadvertently left a revolver a little too long after using 777 and it was a rusty mess, about the same as it would have been with BP or Pyrodex.
Surprisingly so.